This invention relates to thermal overload relays such as that described in K. A. Forsell et al U.S. Pat. No. 4,528,539, issued July 9, 1985, assigned of this application, which patent is incorporated herein by reference. The thermal overload relay disclosed in that patent comprises a compartmented housing wherein thermally responsive bimetal members are positioned in the compartments, extending to a differential operating mechanism disposed in the lower portion of the relay housing. The differential operating mechanism comprises a driven slide bar and a follower slide bar interconnected by a crank pivotally attached to each respective slide bar on different axes. The relay features cassettes which mount in the respective narrow compartments of the housing, the cassettes each having a heater which is respectively connected in one phase of a plural-phase power supply, the heater being disposed in predetermined proximity to the associated bimetal element in the respective compartment. The relay also comprises a switch which is actuated by the differential operating mechanism crank. Associated with the switch is an ambient compensator which functions to arrest movement of the follower slide bar at a predetermined point to thereafter achieve differential movement between the driver slide bar and follower slide bar which effects switch actuating rotation of the crank. The ambient compensator comprises a bimetal element which extends into the path of the follower slide bar and deflects in the same direction as the bimetal elements in each compartment to nullify the affect of ambient temperature on the switch operating mechanism. The position of the ambient compensator is also manually adjustable to provide a range of current values at which the thermal overload relay will operate for each particular heater selection. The adjustment is accomplished by affixing the ambient compensating bimetal to an adjustment mechanism having one adjustment for factory calibration and another adjustment which is user operable to select a particular current setting from a current range. The adjustment mechanism has numerous parts, two pivot points, a factory calibration adjustment screw arrangement which must accommodate lateral motion of the screw, and a spring that tends to separate the housing base and cover. Although this mechanism has performed satisfactorily and is well suited for its intended purpose, the present invention relates to improvements thereover.